Structure of hot dense matter at subnuclear densities is investigated by quantum molecular dynamics ( QMD ) simulations .
We analyze nucleon distributions and nuclear shapes using two-point correlation functions and Minkowski functionals to determine the phase-separation line and to classify the phase of nuclear matter in terms of the nuclear structure .
Obtained phase diagrams show that the density of the phase boundaries between the different nuclear structures decreases with increasing temperature due to the thermal expansion of nuclear matter region .
The critical temperature for the phase separation is \gtrsim 6 MeV for the proton fraction x = 0.5 and \gtrsim 5 MeV for x = 0.3 .
Our result suggests the existence of “ spongelike ” phases with negative Euler characteristic in addition to the simple “ pasta ” phases in supernova cores until T \lesssim 3 MeV .